Conventional high strength stainless steels are roughly classified into (1) martensitic stainless steels, (2) work-hardenable austenitic stainless steels, and (3) precipitation-hardenable stainless steels.
Martensitic stainless steels mainly comprise Fe-Cr-C system and are substantially of single austenitic phase at the quenching temperature (which is 900.degree.-1100.degree. C., but varies depending on the content of Cr and C), but their martensite start point (Ms point) is higher than the room temperature range and they are so-called quench-hardenable steels.
These steels are hard and poor in workability in the quenched state or the quenched and tempered state. Therefore, in these steels, working such as bending, machining and cutting is carried out in the annealed state and high strength is provided by a heat-treatment such as quenching and tempering after the steel is shaped as desired. However, heat-treatment of large parts or members is difficult, and these steel materials are susceptible to weld cracking, and, therefore, tempering must be carried out after welding.
When martensitic stainless steels are to be used as structural members, the above-mentioned defects must be compensated for. To this end, a steel in which the C content is restricted lower so that a massive martensite phase appears in the quenched state has been considered. The steel of Japanese Patent Publication No. 51-35447 (1976) is an example of such a steel. A steel which falls within the claim of said patent publication is presented in No. 33 of "Nisshin Seiko Giho (Technical Reports of Nisshin Steel Co.)" (December 1975 issue). The composition thereof is: C: 0.032%, Si: 0.75%, Mn: 0.14%, Ni: 4.01%, Cr: 12.4%, and Ti: 0.31%. This material has a tensile strength of about 108 kgf/mm.sup.2 and an elongation of about 6%, and that is very low in weld softening. Although low weld softening and high tensile strength are desirable for a welded structural material, the steel is still unsatisfactory as a structural material to be worked since elongation is poor and cracking easily occurs even in light working.
Work-hardenable austentic stainless steels have the metastable austenitic phase as represented by AISI 301, 201, 304, 202, etc., and are hardened by cold working. Mechanical properties attained by this cold working are stipulated in JIS G 4307. For instance, in 1/2H of AISI 301, it is specified that yield strength is not less than 77 kgf/mm.sup.2, tensile strength is not less than 105 kgf/mm.sup.2 and elongation is not less than 10%. That is, both tensile strength and elongation are specified as being high. However, the materials of this class have a defect in that when they undergo heat input such as welding, the heated part or weld softens. Also in some cases, chromium carbide deposit in the part heated by welding, and chromium-poor layers are formed and thus intergranular stress corrosion cracking occurs.
Precipitation-hardenable stainless steels are classified into martensite type, ferrite type and austenite type in accordance with the structure of the matrix. But all of them contain at least one of Al, Ti, Nb, Cu, Mo, V, etc., which contribute to age-hardening, and the steels are hardened by precipitation of intermetallic compounds caused by aging from the super-saturated solid-solution state. These steels have a tensile strength of 140-190 kgf/mm.sup.2 and an elongation of 2-5%, depending upon the state of the matrix, contents of the elements which contribute to age-hardening, etc.
When these steels are used for structural members, generally working and welding are effected prior to age-hardening. However, it is difficult to age-harden larger structural members.
As has been described, the materials conventionally known as high strength stainless steels do not possess all of strength, workability and resistance to weld softening.
The object of the present invention is to provide a novel high strength steel material free from the above-described defects. The object is achieved by heating a steel material of a martensitic structure, which is in a specific composition range and that satisfies a specific composition relationship, to cause reverse austenitic transformation and stabilize the thus formed reverse-transformed austenite phase.